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Experimental evidence is presented for the effect of the background gas on the expansion of the metallic plasma cloud produced at the cathode spot of an arc (50 A, 25-300 Â¿s, 0-50 torr He) with or without an applied magnetic field (850 G). Streak and frame high-speed photography and electrostatic probes were used. It appears that the background gas is swept up by the expanding plasma slowing it down. A snow-plow model is derived which accounts satisfactorily for the results.